Liquid crystal mixture

ABSTRACT

Liquid crystal mixture comprising one or several compounds of formula (I) wherein R 1 : is H or a linear or branched alkyl group of 1 to 20 or a linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case optionally one —CH 2 — group being replaced by cyclohexylen-1,4-diyl, or one or two —CH 2 — groups optionally being replaced by —O—, if non-adjacent to N, or by —C(═O)—, —Si(CH 3 ) 2 —; and/or one or more H of the alkyl or alkenyl group optionally being replaced by F or CH 3  R 2 : stands for a) H or F b.) a linear or branched alkyl group of 1 to 20 or linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case one or two —CH 2 — groups optionally being replaced by —O—, —C(═O)O—, —Si(CH 3 ) 2 —, and/or one or more fl of the alkyl or alkenyl group being replaced by F or CH 3  c) a radical (a) wherein independently from the respective meanings in (1) R 3 , R 4 , R 5 , R 6 : are independently from another an alkyl group of 1 to 8 C atoms M 1 , M 2 : represent independently from another a single bond, —OC(═O), —C(═O)O—, —OCH 2 —, —NH— A: is a) a linear or branched alkan-α,ω)-diyl group of 1 to 20 or alkene-α,ω-diyl group of 2 to 20 C atoms, optionally, if non-adjacent to M 1/2 , one or two non-adjacent —CH 2 — groups may be replaced by —O— b) the group —C(═Y)— wherein Y is CH-Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M 1  and M 2  are —C(═O)O— and —OC(═O)—, c) the group —CHY wherein Y is CH 2 -Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M 1  and M 2  are —C(═O)O— and —OC(═O)—, d) a group (b) wherein p, q are 0, 1 or 2, the sum of p+q being ≧1; M 3  is a single bond or —OC(═O)—, —C(═O)O—, —OCH 2 —, —CH 2 O—, —C≡C—, —CH 2 CH 2 —, —CH 2 CH 2 CH 2 CH 2 — the radicals (c) and (d) are independently from another phenylen-1,4-diyl, which optionally can be substituted by one, two or three F, or cyclohexan-1,4diyl, which can optionally be substituted by one CN, CH 3  or F, or pyrimidin-2,5-diyl, optionally substituted by one F, pyridine-2,5-diyl, which can optionally be substituted by one 1-, or naphthalene-2,6-diyl, which can optionally be substituted by one, two or three F, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionally substituted by one, two or three FD, or decahydronaphthalene-2,6-diyl, or indane-2,5(6)-diyl, or tluorene-2,-7-diyl, or phenanthrene-2,7-diyl, or 9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or (1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl, or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, or piperazin-1,4-diyl X is H, OH, a linear or branched alkyl or alkyloxy group of 1 to 20 C atoms, wherein one or two —CH 2 — groups can be replaced by —O—, —C(═O)O—, —Si(CH 3 ) 2 —, optionally one or more 1-1 being replaced by F or CH 3  m: is 0 or 1 X and M 1 -(A) m -M 2 -R 2  together can constitute a) a ring of 4 to 16 members, optionally substituted by an alkyl radical of 1 to 15 C atoms b) a combination of two either directly linked or spiro-linked rings of independently from another 4 to 16 members, optionally substituted by an alkyl radical of 1 to 15 C atoms in either of the three variants the rings independently from another being carbocycles or carbocycles comprising B, N, O or S heteroatoms.  
     Liquid crystal mixture comprising compounds of formula (I) wherein  
                 
 
     R 1 : is H or a linear or branched alkyl group or branched alkenyl group  
     R 2 : stands for  
     a) H or F  
     b) a linear or branched alkyl group or a linear or branched alkenyl group  
     c) a radical  
                 
 
     wherein  
     R 3 , R 4 , R 5 , R 6 : are an alkyl group  
     M 1 , M 2 : represent independently from another a single bond, —OC(═O), —C(═O)O—, —OCH 2 —, —NH— 
     A: is  
     a) a linear or branched alkan-α,ω-diylgroup or alkene-α,ω-diyl group  
     b) the group —C(═Y)— wherein Y is CH-Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups,  
     c) the group —CHY wherein Y is CH 2 -Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups  
     d) a group  
                 
 
     wherein  
     p, q are 0, 1 or 2;  
     M 3  is a single bond or —OC(═O)—, —C(═O)O—, —OCH 2 —, —CH 2 O—, —C≡C—, —CH 2 CH 2 —, —CH 2 CH 2 CH 2 CH 2 — 
     the radicals  
                 
 
     are phenylen-1,4-diyl,  
     X is H, OH, a linear or branched alkyl or alkyloxy group,  
     m: is 0 or 1  
     X and M 1 -(A) m -M 2 -R 2  together can constitute  
     a) a ring of 4 to 16 members,  
     b) a combination of two either directly linked or spiro-linked rings.

[0001] The present invention relates to a novel liquid crystal mixture. More particularly, it relates to a chiral smectic or ferroelectric liquid crystal mixture, which is chemically very stable against heat and light, thus being especially suitable for active matrix panels. A further aspect of this invention is the use of this liquid crystal mixture in displays, especially active matrix displays. Yet another aspect of this invention are active matrix displays comprising such a mixture.

[0002] Since Clark and Lagerwall found Surface Stabilized Ferroelectric Liquid Crystals (SSFLC) in 1980 [N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 36, 899 (1980)], these liquid crystals have attracted attentions as display materials in the coming generation and a number of studies have been carried out thereon.

[0003] There are two major advantages of FLC displays: the ‘pixel speed’ is easily 100 or even 1000 times faster than nematics and secondly, due to the bistability, the resolution of such displays is not limited by the material.

[0004] However, passive matrix FLC displays are limited by certain trade-off relationships: the higher the number of scanning lines, the higher is the frame addressing time and thus the speed of a display is always in competition with its resolution.

[0005] With respect to the trade-offs of passive matrix FLC displays several authors have suggested to combine the active matrix with FLC. The first approach was made by Hartmann (IEEE Trans. Electron. Devices 1989, 36(9), part 1, pp. 1895-9) in the 805 utilising the charge-controlled bistability of a quasi-bookshelf FLC with MOS-FET technology. The large Ps values, however, prohibited higher resolution with a-Si TFT driving.

[0006] Takatoh et. al. (6^(th) International Conference on Ferroelectric Liquid Crystals, 1997, 20-24 July, Brest, France; M. Takatoh et al. 1998, SID Digest, 1171-1174) have demonstrated an AM display based upon chiral smectics using a very high Ps material driven with an active matrix with polycrystalline Silicon-TFT. Nito et. al. (Nito et al., 1993, Journal of the SID, ½, 163-169.) have suggested a monostable AM-FLC with much lower Ps, however, with the disadvantage of a stripey FLC texture which is not suitable for high contrast displays without further improvements. Furue et. al. (Furue, H. et al., 1998, IDW '98, 209-212) suggested a polymer stabilized SSFLCD with a FELIX® mixture with a material having a moderate P_(S) value.

[0007] High P_(S)-values require long charge-up times on each of the pixels and are therefore incompatible with high resolution, i.e. a large number of scanning lines. This is the reason why Takatoh et. al. have used a special poly-crystalline silicon active matrix that allows for higher current densities as compared to the standard and cost-effective amorphous silicon TFT.

[0008] Asao et. al. have presented a monostable FLC mode (Y. Asao et al., ILCC 2000, Sendai, and Jpn. J. Appl. Phys. 38, L534-L536, 1999 therein called “half-V-shape FLC” mode; see also T. Nonaka et. al., Liquid Crystals 26(11), 1599-1602, 1999, therein called “CDR” mode). Such displays provide, by virtue of their smaller Ps values, solutions for the gray scale problem and the resolution limitation caused by too large Ps values in active matrix panels.

[0009] A remaining problem in the application of TFT-LCD (e.g. monostable FLC) is the limited “holding ratio”, caused by the activity of the manifold of charge carriers that are present in the pixel volume and which tend to dis-charge the pixel within too short a time. In particular for the fast switching displays with a very thin cell gap the problem of ionic charges which build up depolarisation fields leads to significant limitations (cp. Sasaki, Japan Display 1986,62; Nakazono, Int. Dev. Res. Cent. Techn. Rep. IDRC 1997, 65; Naemura, SID Dig. Techn. Pap. 1989, 242; Fukuoka, AM LCD 1994, 216; Takatori, AM-LCD 97 DIGEST. 1997, 53; Takatoh, Polym. Adv, Technol. 11, 413 (2000)).

[0010] Thus it is an important factor to maintain a very low ion content and to achieve chemical stability against heat and light, both of which could cause additional ion formation. This demand has in practice lead to the exclusion of any material comprising hetero atoms such as N, S, even 0 in liquid crystal mixtures for active matrix (i.e. TFT or MIM) applications {cp. e.g. Petrov et al., Liq. Cryst. 19(6), 729 (1995) [CAN 124:101494]; Petrov, Proc. SPIE-Int. Soc. Opt. Eng.(1995), 2408 [CAN 123:241500]; Dabrowski, Biul. Wojsk. Akad. Techn. 48(4), 5 (1999) [CAN 131:163227]; Kirsch, Angew. Chem., Int. Ed. 39(23),4216 (2000) and references cited in these papers).Whereas, so far, this could successfully be done for nematics (cp. e.g. DE-A 1 962 9812, p. 12 to 16), there is hardly any smectic (S_(c)) material without such hetero atoms (cp. Demus et al., Flussige Kristalle in Tabellen, vol. 1 and 2). Thus the use of fast switching smectics for TFT application is strongly limited if not prohibited and the range of potentially available nematics is strongly reduced.

[0011] The object of the present invention was therefore to provide a liquid crystal mixture with very high resistivity or holding ratio irrespective of the comprisment of hetero atoms, especially a chiral smectic, in particular ferroelectric or antiferroelectric liquid crystal mixtures which are suitable for active matrix panels, especially with respect to withstanding heat- or light-induced chemical stress while maintaining the performance of a low-ion-content mixture.

[0012] The present invention provides a liquid crystal mixture, especially a chiral smectic D mixture, in particular a ferroelectric or antiferroelectric liquid crystal mixture, most particularly a monostable ferroelectric liquid crystal mixture comprising one or several compounds of formula (I)

[0013] wherein

[0014] R¹: is H or a linear or branched alkyl group of 1 to 20 C atoms or linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case one —CH₂— group can be replaced by cyclohexylen-1,4-diyl, or one or two —CH₂— groups can be replaced by —O—, if non-adjacent to N, or by —C(═O)— or —Si(CH₃)₂—, and one or more H of the alkyl or alkenyl group optionally being replaced by F or CH₃.

[0015] R²: stands for

[0016] a) H or F

[0017] b) a linear or branched alkyl group of 1 to 20 C atoms or linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case one or two —CH₂— groups can be replaced by —O—, —C(═O)O—, —Si(CH₃)₂—, it also being possible to replace one or more H of the alkyl or alkenyl group by F or CH₃.

[0018] c) for a radical

[0019] wherein independently from the respective meanings in (I)

[0020] R³, R⁴, R⁵, R⁶: represent independently from another an alkyl group of 1 to 8 C atoms

[0021] M¹, M² stand: independently from another for a single bond, —OC(═O), —C(═O)O—, —OCH₂—, or —NH—

[0022] A: is

[0023] a) a linear or branched alkan-α,ω-diylgroup of 1 to 20 or alkene-α,ω-diyl group of 2 to 20 C atoms, it also being possible to replace, if non-adjacent to M^(1/2) one or two non-adjacent —CH₂— groups by —O—

[0024] b) the group —C(═Y)— wherein Y is CH-Z with Z being phenylen-1,4-diyl, optionally substituted by 1 to 3 halogen atoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—, respectively;

[0025] c) the group —CHY wherein Y is CH₂-Z with Z being phenylen-1,4-diyl, optionally substituted by 1 to 3 halogen atoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—, respectively

[0026] d) a group

[0027] wherein

[0028] p, q are 0, 1 or 2, and the sum of p+q being ≧1

[0029] M³ is a single bond, —OC(═O)—, —C(═O)O—, —OCH₂—, —CH₂O—, —C≡C—, —CH₂CH₂—, —CH₂CH₂CH₂CH₂—

[0030] the radicals

[0031] are independently from another phenylen-1,4-diyl, which optionally can be substitued by one, two or three F, or cyclohexan-1,4-diyl, which can optionally be substituted by one CN, CH₃ or F, or for pyrimidin-2,5-diyl, optionally substituted by one F, pyridine-2,5diyl, which can optionally be substituted by one F, or naphthalene-2,6-diyl, which can optionally be substituted by one, two or three F, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionally substituted by one, two or three F), or decahydronaphthalene-2,6diyl, or indane-2,5(6)-diyl, or fluorene-2,-7-diyl, or phenanthrene-2,7-diyl, or 9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or (1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl, or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, or piperazin-1,4-diyl

[0032] X is H, OH, or a linear or branched alkyl or alkyloxy group of 1 to 20 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —C(═O)O—, —Si(CH₃)₂—, it also being possible to replace one or more H by F or CH₃

[0033] m: is 0 or 1

[0034] X and M¹-(A)_(m)-M²-R² together can constitute

[0035] a) a ring of 4 to 16 members, optionally substituted by alkyl of 1 to 15 C atoms

[0036] b) a combination of two either directly linked or spiro-linked rings of independently from another 4 to 16 members, optionally substituted by alkyl of 1 to 15 C atoms in either of the three variants the rings are independently from another carbocycles or carbocycles comprising B, N, O or S heteroatoms.

[0037] Preferably the mixtures comprise 0.01 wt.-% to 10 wt.-% of one or several compounds of formula (I).

[0038] Especially prefered are mixtures comprise 0.1 wt.-% to 5 wt.-% of one or several compounds of formula (I).

[0039] Preferably the mixtures comprise one or several compounds of the following formulae

[0040] wherein

[0041] X is H

[0042] R¹ is H or alkyl of 1 to 4 C atoms

[0043] R³, R⁴, R⁵, R⁶ are CH₃

[0044] and/or

[0045] wherein

[0046] X is H

[0047] R¹ is H or alkyl of 1 to 4 C atoms

[0048] R³, R⁴, R⁵, R⁶ are CH₃

[0049] and/or

[0050] wherein

[0051] X is H

[0052] R¹ is H or alkyl of 1 to 4 C atoms

[0053] R³, R⁴, R⁵, R⁶ are CH₃

[0054] and/or

[0055] wherein

[0056] X is H

[0057] R¹ is H or alkyl of 1 to 4 C atoms

[0058] R³, R⁴, R⁵, R⁶ are CH₃.

[0059] Particularly prefered are mixtures comprising

[0060] wherein

[0061] X is H

[0062] R¹ is H or alkyl of 1 to 4 C atoms

[0063] R³, R⁴, R⁵, R⁶ are CH₃

[0064] M¹ is —OC(═O)—

[0065] M² is —C(═O)O—

[0066] n is 4 to 12

[0067] and/or

[0068] wherein

[0069] X is H

[0070] R¹ is H or alkyl of 1 to 4 C atoms

[0071] R³R⁴, R⁵, R⁶ are CH₃

[0072] R² is H

[0073] M¹ is —OC(═O)—

[0074] M² is a single bond

[0075] n is 8 to 20

[0076] and/or

[0077] wherein

[0078] X is H

[0079] R¹ is H or alkyl of 1 to 4 C atoms

[0080] R³, R⁴, R⁵, R⁶ are CH₃

[0081] R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂—, it also being possible to replace one or more H by F or CH₃

[0082] M¹ is —OC(═O)—

[0083] M¹ is a single bond

[0084] is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl

[0085] and/or

[0086] wherein

[0087] X is H

[0088] R¹ is H or alkyl of 1 to 4 C atoms

[0089] R³,R⁴, R⁵, R⁶ are CH₃

[0090] R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂—, it also being possible to replace one or more H by F or CH₃

[0091] M¹ is —OC(═O)—

[0092] M² is a single bond

[0093] M³ is a single bond, —OC(═O)—, —OCH₂—

[0094] is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl

[0095] is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl

[0096] and/or

[0097] wherein

[0098] X is H

[0099] R¹ is H or alkyl of 1 to 4 C atoms

[0100] R³, R⁴, R⁵, R⁶ are CH₃

[0101] R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂—, it also being possible to replace one or more H by F or CH₃

[0102] M¹ is a single bond

[0103] M² is a single bond

[0104] is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl,

[0105] and/or

[0106] wherein

[0107] X is H

[0108] R¹ is H or alkyl of 1 to 4 C atoms

[0109] R³, R⁴, R R⁶ are CH,

[0110] R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂—, it also being possible to replace one or more H by F or CH₃

[0111] M¹ is a single bond

[0112] M² is a single bond

[0113] M³ is a single bond

[0114] is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl,

[0115] Several compounds of the formula (I) are commercially available. The synthesis of the not commercially available compounds of formulae (I) and (II), resp., or new D compounds of formulae (I), (II), (III) and (IV), resp., are performed in analogy to pertinent papers, e.g. Dagonneau et al., Synthesis 1984, pp. 895-916 [CAN 103:372941; Rozantsev et al., ACS Symp. Ser. (1985), 280 (Polym. Stab. Degrad.), pp. 11-35 [CAN 103:142668].

[0116] In particular the commercially available 2,2,6,6-tetramethyl-piperidine-4-one and 4-hydroxy-2,2,6,6-tetramethylpiperidine can serve as precursors for the new compounds of formulae (I), (II), (Ill) and (IV).

[0117] E.g. (Ia), wherein M¹ and M² are single bonds, R² an alkyl group of 1 to 20 C atoms, can be obtained by reacting 2,2,6,6-tetramethyl-4-piperidone with an alkyl D magnesium halide to render 4-alkyl-4-hydroxy-2,2,6,6-tetramethyl-piperidine (in analogy to Skowronski et al., Pol. J. Chem. 54, 195, 1980)), dehydration to 3,4-dehydro-4-alkyl-2,2,6,6-tetramethylpiperidine [corresponds to formula (IV)] and consecutive hydrogenation (e.g. in analogy to DE-A 2258086). Alternatively Wittig reagents can be applied for the first step (e.g. in analogy to Collum et al., J. Am. Chem. Soc. 113, 9575 (1991)).

[0118] E.g. (Ia), wherein M¹ is —OC(═O)—, can be obtained by subjecting 4-hydroxy-2,2,6,6-tetramethylpiperidine to esterification (in analogy to U.S. Pat. No. 4,038,280, Example 14) with an appropriate carboxylic acid derivative X—C(═O)(CH₂)-M²-R² (X: Cl, Br or OH).

[0119] E.g. (Ia1), wherein M¹ is —OC(═O)— and M² is —C(═O)O—, can be obtained by subjecting 4-hydroxy-2,2,6,6-tetramethylpiperidine to esterification (in analogy to U.S. Pat. No. 4,038,280, Example 14) with an appropriate carboxylic acid derivative X—C(═O)(CH₂)C(═O)X (X: Cl, Br or OH).

[0120] E.g. (Ib), wherein M¹ is a single bond, M² is —OCH₂—, R² an alkyl group of 1 to 20 C atoms and

[0121] is phenylen-1,4-diyl, can be obtained by bringing into reaction 2,2,6,6-tetramethyl-piperidine-4-one and phenol (in analogy to SU 631 516; CA 90:54839) to render 3,4-dehydro-4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine, which is hydrogenated to 4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine (in analogy to DE-A-2 258 086). This material can be (in analogy to U.S. Pat. No. 4,038,280, Example 25) transfered to the above mentioned examples of (Ib) by reacting with an appropriate alkyl halide X—R² (X: Halide, Tosylate, Mesylate; R² not H or F).

[0122] E.g. (Ib), wherein M¹ is a single bond, M² is —OC(═O)—, R² an alkyl group of 1 to 20 C atoms and

[0123] is phenylen-1,4-diyl, can be obtained from above mentioned 4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine by esterification with an appropriate carboxylic acid derivative X—C(═O)—R² (X: Cl, Br, OH; R² not H or F) (e.g. in analogy to U.S. Pat. No. 4,038,280, Example 14).

[0124] E.g. (Ib), wherein M¹ is a single bond, M² is —OC(═O)— or —OCH₂—, R² an alkyl group of 1 to 20 C atoms and

[0125] is cyclohexyl-1,4-diyl, can be obtained from above mentioned 4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine by hydrogenation (in analogy to DE-A-2 415 818) to 4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine and consecutive etherification with an appropriate alkyl halide X—R² (X: Halide, Tosylate, Mesylate; R² not H or F) or esterification with an appropriate carboxylic acid derivative X—C(═O)—R² (X: Cl, Br, OH; R² not H or F) (e.g. in analogy to U.S. Pat. No. 4,038,280, Example 14 and 25), resp.

[0126] E.g. (Ic), wherein M¹ is a single bond, M³ is —OC(═O)—, R²H, F or an alkyl group of 1 to 20 C atoms,

[0127] is phenylen-1,4-diyl or cyclohexyl-1,4-diyl and

[0128] is phenylen-1,4-diyl, optionally substituted by one, two or three F, or cyclohexyl-1,4-diyl, optionally substituted by one CN, CH₃ or F, or pyrimidin-2,5-diyl, pyridin-2,5-diyl, naphthalin-2,6-diyl, optionally substituted by one, two or three F, thiophen-2,4-diyl, thiophen-2,5-diyl, can be obtained by subjecting above mentioned 4-(4-hydroxy-phenyl)-2,2,6,6-tetramethyl-piperidine and 4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine, respectively, to esterification with a moiety (X: F, C), Br or OH) (e.g. in analogy to U.S. Pat. No. 4,038,280, Example 14).

[0129] E.g. (Ic), wherein M¹ is a single bond, M³ is —OCH₂—,

[0130] is phenylen-1,4-diyl or cyclohexyl-1,4-diyl and

[0131] is phenylen-1,4-diyl, optionally substituted by one, two or three F, or cyclohexyl-1,4-diyl, optionally substituted by one CN, CH₃ or F, or pyrimidin-2,5-diyl, pyridin-2,5-diyl, naphthalin-2,6-diyl, optionally substituted by one, two or three F, or thiophen-2,4-diyl, thiophen-2,5-diyl, can be obtained by subjecting (in anlogy to U.S. Pat. No. 4,038,280, Example 25) above mentioned 4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine and 4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine, respectively, to etherification with a moiety (X: Cl, Br, Tosylate, Mesylate)

[0132] E.g. (Ic), wherein M¹ is a single bond, M³ is a single bond, R²H, F or an alkyl group of 1 to 20 C atoms,

[0133] is phenylen-1,4-diyl or cyclohexyl-1,4-diyl and

[0134] is phenylen-1,4-diyl, optionally substituted by one, two or three F, or cyclohexyl-1,4-diyl, optionally substituted by one CN, CH₃ or F, or pyrimidin-2,5-diyl, pyridin-2,5-diyl, naphthalin-2,6-diyl, optionally substituted by one, two or three F, thiophen-2,4-diyl, thiophen-2,5-diyl, can be obtained by subjecting above mentioned 4-(4-hydroxy-phenyl)-2,2,6,6-tetramethyl-piperidine and 4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine, respectively, after transformation into an appropriate derivate (e.g. triflate), to an aryl-(cyclohexyl-)-aryl coupling reaction {Poetsch, Kontakte (Darmstadt), 1988 (2), p.15)} with a moiety (X: ClMg—, BrMg—, IMg—, Li—, ClZn—, (HO)₂B—).

[0135] E.g. (Id) can be obtained in analogy to (Ic), in all the variants given above.

[0136] A further object of the invention is to provide compounds of formula (II) and a nematic liquid crystal mixture comprising at least one compound of formula (I) or (II). Preferably the mixture comprises 0,05 to 5% of one or several compounds of formula (I) and/or (II). Especially prefered are mixtures comprising 0,05 to 5% of one or several compounds of formula (II).

[0137] wherein

[0138] R¹ is H or alkyl of 1 to 12 C atoms

[0139] R² is H, a linear or branched alkyl group of 1 to 16 or linear or branched alkenyl group of 2 to 16 C atoms, wherein in either case one —CH₂— group can be replaced by —O—, it also being possible to replace one or more H by F

[0140] M¹ is —OC(═O)— or a single bond

[0141] M³ is a single bond

[0142] G¹-G² is CH₂—CH— or A-H═C—

[0143] p is 0 or 1

[0144] is phenylen-1,4-diyl, optionally substituted by one or two F, cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by one or two F per ring, 1,1′-cyclohexyl-phenyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-phenylcyclohexyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-bicyclohexyl-4,4′-diyl

[0145] is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, with the proviso

[0146] a) R² is H only in case p is 1

[0147] b) if p is 0, than R² is a linear or branched alkyl group of 1 to 16 or linear or branched alkenyl group of 2 to 16 C atoms and

[0148]  is cydohexylen-1,4-diyl

[0149] c)-G¹-G²- can be —CH═C— only in case M¹ is a single bond

[0150] A further object of the invention is to provide compounds of formula (III) and a chiral smectic liquid crystal mixture comprising at least one compound of formula (III). Preferably the mixture comprises 0,05 to 5% of one or several compounds of formula (III).

[0151] wherein

[0152] R¹ is H or alkyl of 1 to 12 C atoms

[0153] R² is H, a linear or branched alkyl group of 1 to 16 or linear or branched alkenyl group of 2 to 16 C atoms, wherein in either case one —CH₂— group can be replaced by —O—, it also being possible to replace one or more H by F

[0154] M¹ is —OC(═O)— or a single bond

[0155] M³ is a single bond

[0156] G¹-G² is —CH₂—CH— or —CH═C—

[0157] p is 0 or 1

[0158] is pyrimidin-2,5-diyl, pyridin-2,5-diyl, optionally ortho to N substituted by F, phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by one or two F per ring, 1,1′-cyclohexylphenyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-phenylcyclohexyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-bicyclohexyl-4,4′-diyl

[0159] is phenylen-1,4-diyl, optionally substituted by one or two F, cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by one or two F per ring, 1,1′-cyclohexylphenyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-phenylcyclohexyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-bicyclohexyl-4,4′-diyl, pyrimidin-2,5-diyl, or pyridin-2,5-diyl, optionally ortho to N substituted by F,

[0160] with the provisos

[0161] a) one and only one of

[0162]  or

[0163]  is pyrimidin-2,5-diyl or pyridin-2,5-diyl, optionally ortho to N substituted by F

[0164] b)-G¹-G²- can be A-H═C— only in case M¹ is a single bond

[0165] Yet another object of the invention is to provide a liquid crystal mixture comprising at least 1 compound of formula (IV)

[0166] wherein R¹ to R⁶, A, M² and m have the meanings as in formula (1) and M¹ is a single bond.

[0167] A further object of the invention is to provide a liquid crystal display device, especially one operated in an active matrix panel using the above described mixtures.

[0168] A further object of the invention is to provide a chiral smectic liquid crystal display device, especially one operated in an active matrix panel using the above described mixtures.

[0169] A further object of the invention is to provide a ferroelectric liquid crystal display device, especially one operated in an active matrix panel using the above described mixtures. With preference this display is a monostable ferroelectric display, such as half-V-shape, CDR or short pitch FLC displays.

[0170] A further object of the invention is to provide an antiferroelectric liquid crystal display device, especially one operated in an active matrix panel using the above described mixtures. With preference this display is a monostable antiferroelectric display, such as the so-called “V-shape” mode.

[0171] A further object of the invention is to provide a nematic liquid crystal display device, especially one operated in an active matrix panel using the above described mixtures.

[0172] Yet a further object of the invention is the use of the above described mixtures in a liquid crystal display, especially one operated in an active matrix panel, especially if the liquid crystal is a chiral smectic, particularly a monostable chiral smectic mode.

[0173] The liquid crystal mixtures according to the invention are prepared in a manner which is customary per se. As a rule the components are dissolved in one another, advantageously at elevated temperatures.

[0174] The liquid crystal mixtures according to the invention generally comprises of at least two (2), preferably at least five (5), particularly at least eight (8) compounds.

[0175] Reference is made to e.g. DE-A-1 985 7352 or DE-A 1 962 9812 (p.12 to 16) for the LC compounds that can be, besides the compounds of formula (I), (II), (Ill) or (IV), the constituents of mixtures according to the invention.

[0176] Optional additional constituents of the mixtures according to invention are materials that increase the light stability (UV stabilizers, e.g. of the “benzophenone” or “benzotriazole” type). Preferably the mixtures may comprise 0.01 wt.-% to 10 wt.-% of one or several UV stabilizers; especially prefered are mixtures containing 0.1 wt.-% to 5 wt.-% of one or several UV stabilizers.

[0177] Optional additional constitutents of the mixtures according to invention are materials that increase the stability against oxidative degradation (antioxidants, e.g. of the “sterically hindered phenol” type). Preferably the mixtures may comprise 0.01 wt.-% to 10 wt.-% of one or several antioxidants; especially prefered are mixtures comprise 0.1 wt.-% to 5 wt.-% of one or several antioxidants. Optionally the mixtures according to invention may comprise a combination of UV stabilizers and antioxidants.

[0178] The mixtures according to the invention can be used in electro-optical or fully optical elements, for example display elements, switching elements, light modulators, shutters, elements for image processing and/or signal processing or generally in the area of nonlinear optics.

[0179] Chiral smectic liquid crystal mixtures according to the invention are particularly suitable for use in electro-optical switching and display devices (displays). These displays are usually constructed in such a way that a liquid crystal layer is enclosed on both sides by layers which are usually, in this sequence starting from the LC layer, at least one alignment layer, electrodes and a limiting sheet (for example of D glass). In addition, they contain spacers, adhesive frames, polarizers and, for color displays, thin color-filter layers or are operated in the sequential backlight technique. Other possible components are antireflection, passivation, compensation and barrier layers and, for active-matrix displays, electric non-linear elements, such as thin-film transistors (TFTS) and metal-insulator-metal (MIM) elements. The structure of liquid crystal displays has already been described in detail in relevant monographs (see, for example, T. Tsukuda, “TFT/LCD Liquid crystal displays addressed by thin fim transistors”, Japanese Technology Reviews, 1996 Gordon and Breach, ISBN 2-919875-01-91.

[0180] The present invention further provides a chiral smectic liquid crystal (FLC) display device comprising the above-mentioned liquid crystal mixture of the present invention between a pair of substrates each comprising an electrode and an alignment layer formed thereon.

[0181] In a preferred embodiment the FLC display is operated in the monostable mode with active matrix panel.

[0182] The present invention further provides a nematic liquid crystal display device comprising the above-mentioned liquid crystal mixture of the present invention between a pair of substrates each comprising an electrode and an alignment layer formed thereon. In a preferred embodiment the display is operated in the ECB mode (EP-A 0474062). IPS mode (Kiefer et al., Japan Display '92, S.547) or VA mode (Ohmura et al., SID 97 Digest, S.845).

[0183] Several documents are cited in this application, e.g. to discuss the state of the art, synthesis of compounds used in the present invention or application of the mixtures according to the invention. All these documents are hereby incorporated by reference.

EXAMPLES

[0184] Cell Fabrication

[0185] A solution of LQT 120 (Hitachi Kasei) is applied onto glass substrates with ITO by spin coating at 2500 rpm. The substrates are heated at 200° C. for 1 hour to form a film. After rubbing the coated film with a nylon cloth in one direction, the substrates are assembled into a cell with spacers having a thickness of 2.0 μm inserted between the substrates in such a manner that the rubbing directions are anti-parallel to each other. The properties of the liquid crystal mixture are measured by filling the liquid crystal mixture into the cell in the isotropic phase, cooling progressively through the nematic, (smectic A phase in case the liquid crystal mixture has one) and the smectic C phase and then applying a rectangular wave pulse (60 Hz) with varying applied voltage (0 to 10V) to the cell at 25° C. Resistivity and ion-induced spontaneous polarization are measured by means of a MTR-1 device (Toya Technica).

[0186] Table 1 summarizes the results thus obtained by adding small quantities of several types of (I) to S_(c) mixture A and B, respectively. As can be seen by comparing with the respective reference examples rA/rB {comprising no (I)}, the mixtures according to invention surprisingly exhibit a higher resisitivity and decreased ion content. Hence the mixtures according to invention can find application in active-matrix devices. TABLE 1 Mixture A Mixture B Ion Ion wt. Resistivity Cont. wt. Resistivity Cont. ex. % [TΩcm] [nC/cm²] % [TΩcm] [nC/cm²] rA/ Without (I) Trade Name 0 8.5 0.44 0 7.1 2.3 rB 1A 1B

Sanduvor 3052Liq. 0.1 >10 <0.1 0.1 7.22 0.24 2A 2B

Hostavin N20 (Sanduvor 3051 PDR) 0.1 >10 <0.1 0.1 >10 0.25 3A 3B

VP Sanduvor PR-31 0.1 >10 <0.1 0.1 3 0.24 4A 4B

TINUVIN□765 1.0 >10 <0.1 1.0 8.4 0.15 5A 5B

TINUVIN□770 1.0 >10 <0.1 1.0 8.2 0.18 6A with n = 13 to 16 LICOVIN 845 0.1 >10 <0.1 0.1 8.0 <0.1 6B

Example 7

[0187] 2,2,6,6-Tetramethyl-piperidine-4-(4-octyloxy)benzoate was obtained by esterifying 4-hydroxy-2,2,6,6-tetramethyl-piperidine with 4-octyloxybenzoyl chloride in analogy to U.S. Pat. No. 4,038,280; m.p. 79-81° C.

Example 8

[0188] 2,2,6,6-Tetramethyl-piperidine-4-(4′-octylbiphenyl-4-yl)carboxylate) was obtained in analogy to example 7 from 4′-octyl-biphenyl-4-yl carboxylic acid chloride; m.p. 104-106° C.

Example 9

[0189] 2,2,6,6-Tetramethyl-piperidine-4-(trans-4-pentylcyclohexyl)carboxylate was obtained in analogy to example 7 from trans-4-pentylcyclohexyl)carboxylic acid chloride; viscous liquid, crystallizes on standing (m.p. 44 to 48° C.).

Example 10

[0190] 2,2,6,6-Tetramethyl-piperidine-4-[4-(trans-4-propylcyclohexyl)]benzoate was obtained in analogy to example 7 from 4-(trans-4-propylcyclohexyl)benzoyl chloride.

Example 11

[0191] 2,2,6,6-Tetramethyl-piperidine-4-[4-(5-hexyl-pyrimidin-2-yl)]benzoate was obtained in analogy to example 7 from 4-(5-hexyl-pyrimidin-2-yl)benzoylchloride.

Example 12

[0192] 2,2,6,6-Tetramethyl-piperidine-4-[4′-(4-decyloxy)benzoyloxy-biphenyl-4-yl]carboxylate was obtained in analogy to example 7 from [4′-(4-decyloxy)benzoyloxy-biphenyl-4-yl]carboxylic acid chloride.

Example 13

[0193] 2,2,6,6-Tetramethyl-piperidine-4-(4-hexyl)benzoate was obtained in analogy to RO 92779 B1 (CAN 109:171562) by heating a xylene solution of methyl 4-hexylbenzoate, 4-hydroxy-2,2,6,6-tetramethylpiperidine and sodium methanolate to reflux for 10 hrs. Silica treatment and recrystallization rendered pure material of m.p. 61-62° C.

Example 14

[0194] 2,2,6,6-Tetramethylpiperidine-4-(4′-heptylbiphenyl-4-yl)carboxylate was obtained in analogy to example 13 by subjecting methyl 4′-heptylbiphenyl-4-carboxylate to the reaction with 4-hydroxy-2,2,6,6-tetramethylpiperidine; m.p. 106-107° C.

Example 15

[0195] 1,2,2,6,6-Pentamethyl-4-[trans-4-(4-propylcyclohexyl)phenyl]piperidine was obtained in analogy to Skowronski et al., Pol. J. Chem. 54,195,(1980) by reacting in tetrahydrofurane 1,2,2,6,6-pentamethyl-piperidine-4-one with the Grignard reagent prepared from 4-(trans-4-propylcyclohexyl)bromobenzene, subjecting the crude reaction product to azeotropic dehydration to 3,4-dehydro-1,2,2,6,6-pentamethyl-4-[trans-4-(4-propylcyclohexyl)phenyl]piperidine and hydrogenation in tetrahydrofuran, catalyzed by Pd/C, at ambient temperature and atmospheric pressure, followed by silica treatment, as a viscous liquid. ¹H-NMR (300 MHz, CDCl₃/DMSO/TMS) δ=7.18 m, 4H), 2.65-2.49 (m.2H), 2.24 (s.3H), 1.95-1.80 (m,6H), 1.55-1.15 (m,9H), 1.12-0.96 (m,14H), 0.91 (t, 3H)

[0196] Table 2 summarizes the results—(achieved with the equipment and procedure of examples 1 to 6) obtained by adding small quantities of several types of (I) to one of the following Sc mixtures:

[0197] M1 {achiral block mixture based on 5-alkyl-2-(4-alkyloxyphenyl)pyrimidines}

[0198] M2 (chiral mixture based on derivatives of phenylpyrimidines)

[0199] M3b (achiral multi-component mixture based on ester and ether derivatives of ylpyrimidines, fluorinated bi- and terphenyls and sulfur heterocycles) M3 (like M³b, but chiral dopant added) TABLE 2 Ion Content [nc/cm²] storage storage storage storage storage example condition mixture 0 hrs 100 hrs 200 hrs 300 hrs 500 hrs reference.  20 V, 5 Hz M1 <0.01 0.3 0.35 0.48 0.25 16  20 V, 5 Hz M1 + 5% <0.01 <0.01 <0.01 <0.01 <0.01 expl. 13 reference.  20 V, 5 Hz M3b 0.21 4.69 4.36 4.37 4.32 17  20 V, 5 Hz M3b + 0.1% 0.17 0.3 0.48 0.47 0.54 expl. 13 18  20 V, 5 Hz M3b + 0.2% 0.08 0.23 0.23 0.25 0.27 expl. 13 19  20 V, 5 Hz M3b + 0.4% 0.16 0.23 0.2 0.22 0.25 expl. 13 reference. 5 V, 0.1 Hz M4 0 2.56 7.57 3.48 6.33 20 5 V, 0.1 Hz M4 + 0.4% <0.01 <0.01 0.12 0.22 0.17 expl. 13 reference. 5 V, 0.1 Hz M2 0.2 8.18 10.93 9.49 6.43 21 5 V, 0.1 Hz M2 + 0.4% <0.01 <0.01 0.08 0.48 0.73 expl. 13 22 5 V, 0.1 Hz M2 + 0.2% <0.01 1.76 2.37 2.08 1.87 expl. 14 23 5 V, 0.1 Hz M2 + 0.4% <0.01 1.14 1.57 1.41 no data expl. 14 24 5 V, 0.1 Hz M4 + 0.2% <0.01 1.91 2.71 2.79 2.94 expl. 14 25 5 V, 0.1 Hz M4 + 0.4% <0.01 2.02 2.80 3.09 2.91 expl. 14

[0200] As can be seen by comparing with the respective reference examples {comprising no (I)}, the mixtures according to invention surprisingly exhibit a substantially lower ion content [nC/cm²] over a longer period of time. 

1. Liquid crystal mixture comprising one or several compounds of formula (I) wherein

R¹: is H or a linear or branched alkyl group of 1 to 20 or a linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case optionally one —CH₂— group being replaced by cyclohexylen-1,4-diyl, or one or two —CH₂— groups optionally being replaced by —O—, if non-adjacent to N, or by —C(═O)—, —Si(CH₃)₂—; and/or one or more H of the alkyl or alkenyl group optionally being replaced by F or CH₃ R²: stands for a) H or F b) a linear or branched alkyl group of 1 to 20 or a linear or branched alkenyl group of 2 to 20 C atoms, wherein in either case one or two —CH₂— groups optionally being replaced by —O—, —C(═O)O—, —Si(CH₃)₂—, and/or one or more H of the alkyl or alkenyl group being replaced by F or CH₃ c) a radical

wherein independently from the respective meanings in (I) R³, R⁴, R⁵, R⁶: are independently from another an alkyl group of 1 to 8 C atoms M¹, M²: represent independently from another a single bond, —OC(═O), —C(═O)O—, —OCH₂—, —NH— A: is a) a linear or branched alkan-α,ω-diyl group of 1 to 20 or alkene-α,ω-diyl group of 2 to 20 C atoms, optionally, if non-adjacent to M^(1/2), one or two non-adjacent —CH₂— groups may be replaced by —O— b) the group —C(═Y)— wherein Y is CH-Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—, c) the group —CHY wherein Y is CH₂-Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—, d) a group

wherein p, q are 1 or 2, the sum of p+q being ≧1; M³ is a single bond or —OC(═O)—, —C(═O)O—, —OCH₂—, —CH₂O—, —C≡C—, —CH₂CH₂—, —CH?CH₂CH₂CH₂— the radicals

are independently from another phenylen-1,4-diyl, which optionally can be substitued by one, two or three F, or cyclohexan-1,4-diyl, which can optionally be substituted by one CN, CH₃ or F, or pyrimidin-2,5diyl, optionally substituted by one F, pyridine-2,5-diyl, which can optionally be substituted by one F, or naphthalene-2,6-diyl, which can optionally be substituted by one, two or three F, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionally substituted by one, two or three F), or decahydronaphthalene-2,6-diyl, or indane-2,5(6)-diyl, or fluorene-2,-7-diyl, or phenanthrene-2,7-diyl, or 9,10-dihydrophenanthrene-2,7-diyl, or for (1,3,4)thiadiazol-2,5-diyl, or (1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl, or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, or piperazin-1,4-diyl X is H, OH, a linear or branched alkyl or alkyloxy group of 1 to 20 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —C(═O)O—, —Si(CH₃₂—, optionally one or more H being replaced by F or CH₃ m: is 0 or 1 X and M′-(A)_(m)-M²M²-R² together can constitute a) a ring of 4 to 16 members, optionally substituted by an alkyl radical of 1 to 15 C atoms b) a combination of two either directly linked or spiro-linked rings of independently from another 4 to 16 members, optionally substituted by an alkyl radical of 1 to 15 C atoms in either of the three variants the rings independently from another being carbocycles or carbocycles comprising B, N, O or S heteroatoms.
 2. Liquid crystal mixture according to claim 1 wherein the mixture comprises 0.01 wt.-% to 10 wt.-% of one or several compounds of formula (I).
 3. Liquid crystal mixture according to claim 1 and/or 2, comprising at least one compound of the following formulae

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R⁶ are CH, and/or

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R⁶ are CH₃ and/or

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R⁶ are CH₃ and/or

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R⁶ are CH₃
 4. Liquid crystal mixture according to at least one of the preceding claims comprising at least one compound of the following formulae

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R are CH₃ M¹ is —OC(═O)— M² is —C(═O)O— n is 4 to 12 and/or

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R⁶ are CH₃ R² is H M¹ is —OC(═O)— M² is a single bond n is 8 to 20 and/or

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R⁶ are CH₃ R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂, it also being possible to replace one or more H by F or CH₃ M¹ is —OC(═O)— M² is a single bond

is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl and/or

wherein X is H R¹ is H or alkyl of 1 to 4 C atoms R³, R⁴, R⁵, R⁶ are CH₃ R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein one or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂—, it also being possible to replace one or more H by F or CH₃ M¹ is —OC(═O)— M² is a single bond M³ is a single bond, —OC(═O)—, —OCH₂—

is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl

is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl.
 5. Liquid crystal mixture comprising one or several compounds of formula (IV)

wherein R¹ to R⁶, A, M² and m have the meanings as in formula (I) and M¹ is a single bond.
 6. Liquid crystal mixture according to at least one of the preceding claims, wherein the mixture comprises at least two compounds of formula (I) and/or (IV).
 7. Liquid crystal mixture according to at least one of the preceding claims, wherein the mixture comprises additional materials, in particular antioxidants.
 8. Liquid crystal mixture according to claim 7, wherein the mixture comprises 0.01 to 10 wt.-% of one or several antioxidants.
 9. Liquid crystal mixture according to claim 7 and/or claim 8, wherein the mixture comprises besides antioxidants UV-stabilizers.
 10. Liquid crystal mixture according to at least one of the preceding claims, wherein the liquid crystal is a chiral smectic.
 11. Liquid crystal display device comprising a liquid crystal mixture according to at least one of the preceding claims 1 to
 10. 12. Liquid crystal display device according to claim 11, wherein the device is operated in an active-matrix panel.
 13. Liquid crystal display device according to at least one of the preceding claims 11 and/or 12, wherein the display is a ferroelectric or antiferroelectric display or a chiral smectic or a nematic display.
 14. Liquid crystal display device according to at least one of the preceding claims 11 to 13, wherein the display is a monostable display.
 15. Compounds of the formula (II)

wherein R¹ is H or alkyl of 1 to 12 C atoms R² is H a linear or branched alkyl group of 1 to 16 or linear or branched alkenyl group of 2 to 16 C atoms, wherein in either case one —CH₂— group can be replaced by —O—, it also being possible to replace one or more H by F M¹ is —OC(═O)— or a single bond M³ is a single bond G¹-G² is —CH₂—CH— or —CH═C— p is 0 or 1

is phenylen-1,4-diyl, optionally substituted by one or two F, cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by one or two F per ring, 1,1′-cyclohexyl-phenyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-phenylcyclohexyl-4,4′diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-bicyclohexyl-4,4′diyl

is phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, with the provisos a) R² is H only in case p is 1 b) if p is 0, than R² is a linear or branched alkyl group of 1 to 16 or linear or branched alkenyl group of 2 to 16 C atoms and

 is cyclohexylen-1,4-diyl c)-G¹-G²- can be CH═C— only in case M¹ is a single bond
 16. Compounds of the formula (III)

wherein R¹ is H or alkyl of 1 to 12 C atoms R² is H, a linear or branched alkyl group of 1 to 16 or linear or branched alkenyl group of 2 to 16 C atoms, wherein in either case one —CH₂— group can be replaced by —O—, it also being possible to replace one or more H by F M¹ is —OC(═O)— or a single bond M³ is a single bond G¹-G² is —CH₂—CH— or —CH═C— p is 0 or 1

is pyrimidin-2,5-diyl, pyridin-2,5-diyl, optionally ortho to N substituted by F, phenylen-1,4-diyl, optionally substituted by one or two F, or cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by one or two F per ring, 1,1′-cyclohexylphenyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-phenylcyclohexyl-4,4′diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-bicyclohexyl-4,4′ diyl

is phenylen-1,4-diyl, optionally substituted by one or two F, cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by one or two F per ring, 1,1′-cyclohexylphenyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-phenylcyclohexyl-4,4′-diyl, the phenyl moiety optionally substituted by one or two F, or 1,1′-bicyclohexyl-4,4′diyl, pyrimidin-2,5-diyl, or pyridin-2,5-diyl, optionally ortho to N substituted by F, with the provisos a) one and only one of

 is pyrimidin-2,5-diyl or pyridin-2,5-diyl, optionally ortho to N substituted by F b)-G¹-G²- can be —CH═C— only in case M¹ is a single bond 